Data storage devices and methods

ABSTRACT

A data storage device including: a plastic substrate having a plurality of volumes arranged in tracks along a plurality of vertically stacked, laterally extending layers therein; and, a plurality of micro-holograms each contained in a corresponding one of the volumes; herein, the presence or absence of a micro-hologram in each of the volumes is indicative of a corresponding portion of data stored.

RELATED APPLICATION

This application claims priority of U.S. Patent Application Ser. No.60/662,149, filed Mar. 16, 2005, entitled MICRO-HOLOGRAPHIC DATA STORAGEMETHOD AND SYSTEM, AND MICRO-HOLOGRAPHIC RECORDABLE MEDIUM BEINGSUITABLE FOR HIGH DENSITY OPTICAL DATA STORAGE, the entire disclosure ofwhich is hereby incorporated by reference as if being set forth in itsentirety herein.

FIELD OF THE INVENTION

The present invention relates generally to data storage systems andmethods, more particularly to optical based data storage systems andmethods, and holographic data storage systems and methods.

BACKGROUND OF THE INVENTION

Data storage systems and methods are known to be desirable. Volumeholographic recording systems generally use two counter-propagatinglaser or light beams converging within a photosensitive holographicmedium to form an interference pattern. This interference pattern causesa change or modulation of the refractive index of the holographicmedium. Where one of the light beams is modulated, responsively to datato be encoded, the resulting interference pattern encodes the modulatingdata in both intensity and phase. The recorded intensity and phaseinformation may later be detected responsively to reintroduction of theun-modulated, or reference light beam, thereby recovering the encodeddata as reflections.

Conventional “page-based” holographic memories have data written in theholographic medium in parallel, on 2-dimensional arrays or “pages”.

It is desirable to provide a relatively simple, inexpensive and robustholographic memory system. Further, bit-oriented holographic memorysystems are desired.

SUMMARY OF THE INVENTION

A data storage device including: a plastic substrate having a pluralityof volumes arranged along tracks in a plurality of vertically stacked,laterally extending layers; and, a plurality of micro-holograms eachcontained in a corresponding one of the volumes; wherein, the presenceor absence of a micro-hologram in each of the volumes is indicative of acorresponding portion of data stored.

BRIEF DESCRIPTION OF THE FIGURES

Understanding of the present invention will be facilitated byconsidering the following detailed description of the preferredembodiments of the present invention in conjunction with theaccompanying drawings, in which like numerals refer to like parts, and:

FIG. 1 illustrates a configuration for forming a hologram within a mediausing counter-propagating light beams;

FIG. 2 illustrates an alternative configuration for forming a hologramwithin a media using counter-propagating light beams;

FIG. 3 illustrates an alternative configuration for forming a hologramwithin a media using counter-propagating light beams;

FIG. 4 illustrates an alternative configuration for forming a hologramwithin a media using counter-propagating light beams;

FIG. 5 illustrates an alternative configuration for forming a hologramwithin a media using counter-propagating light beams;

FIG. 6 illustrate a light intensity pattern;

FIG. 7 illustrates a refractive index modulation in a linear mediumcorresponding to the intensity pattern of FIG. 6;

FIG. 8 illustrates an expected Bragg detuning of a hologram asdiffraction efficiency being a function of the difference between recordand read temperature;

FIG. 9 illustrates an expected Bragg detuning of a hologram asdiffraction efficiency being as a function of angular change;

FIGS. 10A-10B illustrate a light intensity and corresponding refractiveindex change in a substantially linear optically responsive medium;

FIGS. 10C-10D illustrate a light intensity and corresponding refractiveindex change in a substantially non-linear optically responsive medium;

FIGS. 11A-11B illustrate a light intensity and corresponding refractiveindex change in a substantially linear optically responsive medium;

FIGS. 11C-11D illustrate a light intensity and corresponding refractiveindex change in a substantially non-linear optically responsive medium;

FIG. 12 illustrates an expected micro-hologram reflectivity as afunction of refractive index modulation;

FIGS. 13A and 13B illustrate expected temperature elevation profiles asa function of position, at various times;

FIGS. 14A and 14B illustrate expected refracted index changes as afunction of elevating temperature, and corresponding micro-hologram readand write modes;

FIGS. 15A-15C illustrate expected relationships between light beamincident light beam energy required to elevate material temperature tothe critical temperature as a function of corresponding optical fluenceand normalized linear absorption, light beam waist and distance using areverse saturable absorber, and transmission and fluence using a reversesaturable absorber;

FIGS. 16A and 16B illustrate expected counter-propagating light beamexposures within a media, and corresponding temperature increases;

FIG. 16C illustrates an expected refractive index change correspondingto the temperature increases of FIGS. 16A and 16B;

FIG. 17A illustrates changes in normalized transmission of anortho-nitrostilbene at 25° C. and 160° C. as a function of time;

FIG. 17B illustrates a change in quantum efficiency of anortho-nitrostilbene as a function of temperature;

FIG. 17C illustrates the absorbance of dimethylamino dinitrostilbene asa function of wavelength at 25° C. and 160° C.;

FIG. 18 illustrates a tracking and focus detector configuration;

FIGS. 19A-19C illustrate the contour of a simulated refractive indexprofile;

FIG. 20 illustrates a cross-section of an incident laser beam impinginga region of a holographic recorded media;

FIGS. 21A-21C illustrate near-field distributions (z=−2 μm)corresponding to a simulation of a circular micro-hologram of FIGS.19A-19C;

FIGS. 22A-22C illustrate far-field distributions corresponding to thenear-field distributions of FIGS. 21A-21C, respectively;

FIGS. 23A-23C illustrate the contour of a simulated refractive indexprofile;

FIGS. 24A-24C illustrate near-field distributions corresponding to asimulation of the circular micro-hologram of FIGS. 23A-23C;

FIGS. 25A-25C illustrate far-field distributions corresponding to thenear-field distributions of FIGS. 24A-24C, respectively;

FIGS. 26A-26D illustrate tracking and focus detector configuration andexemplary sensed conditions;

FIG. 27 illustrates a focus and tracking servo system;

FIG. 28 illustrates a formatting having alternating-direction spiraltracks;

FIG. 29 illustrates various track starting an ending points;

FIG. 30 illustrates a formatting including substantially circularmicro-holograms;

FIG. 31 illustrates a formatting including elongated micro-holograms;

FIG. 32 illustrates an off-axis micro-hologram recording;

FIG. 33 illustrates an off-axis micro-hologram reflection;

FIGS. 34A-34C illustrate off-axis micro-hologram recording and reading;

FIG. 35 illustrates a configuration for preparing a mastermicro-holographic media;

FIG. 36 illustrates a configuration for preparing a conjugate-mastermicro-holographic media from a master micro-holographic media;

FIG. 37 illustrates a configuration for preparing a distributionmicro-holographic media from a conjugate master micro-holographic media;

FIG. 38 illustrates a configuration for preparing a distributionmicro-holographic media from a master micro-holographic media;

FIG. 39 illustrates the recording of data by altering a pre-formattedmicro-hologram array; and

FIG. 40 illustrates a configuration for reading a micro-hologram arraybased memory device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the present invention, while eliminating,for purposes of clarity, many other elements found in typicalholographic methods and systems. However, because such elements are wellknown in the art, and because they do not facilitate a betterunderstanding of the present invention, a discussion of such elements isnot provided herein. The disclosure herein is directed to all suchvariations and modifications known to those skilled in the art.

Overview

Volumetric optical storage systems have the potential to fulfill demandsfor high-capacity data storage. Unlike traditional optical disc storageformats, such as compact disc (CD) and digital versatile disc (DVD)formats, where the digital information is stored in a single (or at mosttwo) reflective layer(s), according to an aspect of the presentinvention digital content is stored as localized refractive indexalterations in a plurality of volumes arranged in vertically stacked,laterally directed tracks in the storage medium. Each of the tracks maydefine a corresponding laterally, e.g., radially, directed layer.

According to an aspect of the present invention, single bits, or groupsof bits, of data may be encoded as individual micro-holograms eachsubstantially contained in a corresponding one of the volumes. In oneembodiment, the medium, or media, takes the form of an injectionmoldable thermoplastic disc, and exhibits one or more non-linearfunctional characteristics. The non-linear functional characteristicsmay be embodied as a refractive index change that is a non-linearfunction of experienced energy, such as incident optical intensity orenergy or heating. In such an embodiment, by generating interferencefringes within a given volume of the medium, one or more bits of datamay be selectively encoded in that volume as a later detectablerefractive index modulation. Thus, three-dimensional, molecular,photoresponsive matrix of refractive index changes may thus be used tostore data.

According to an aspect of the present invention, the non-linearfunctional characteristic may establish a threshold energy responsivecondition, below which no substantial change in refractive index occursand above which a measurable change in the refractive index is induced.In this manner, a selected volume can be read, or recovered, byimpinging a light beam having a delivered energy less than thethreshold, and written or erased using a light beam having a deliveredenergy above the threshold. Accordingly, dense matrices of volumes thateach may, or may not, have a micro-hologram substantially containedtherein may be established. Each of the micro-holograms is embodied asan alternating pattern of sub-regions having differing refractiveindices, which correspond to the interference fringes ofcounter-propagating light beams used to write the micro-holograms. Wherethe refractive index modulation decays rapidly as a function of distancefrom a target volume, such as an encoded bit center, the more denselythe volumes may be packed.

According to an aspect of the present invention, the refractive indexchanges in a particular volume may be induced by localized heatingpatterns—corresponding to the interfering fringes of counter-propagatinglaser beams passing through the volume. In one embodiment, therefractive index change results from a density difference between anamorphous and crystalline state of a thermoplastic medium. A transitionfrom one to the other state may be selectively induced in target volumesof a medium by thermally activating sub-volumes of the target volume atinterference fringes therein. Alternatively, the refractive indexchanges may be induced by a chemical change within sub-volumes of targetvolume of the medium, such as a chemical change occurring in a dye orother catalyst within a dye, located within the target volume. Such achemical change may be selectively induced using thermal activation aswell.

A configuration utilizing a non-linearly responsive medium is wellsuited to be used to provide a bit oriented (as opposed to page-based)micro-holographic medium and system that uses a single tightly-focusedlight beam, a focused, slightly focused or unfocused reflected lightbeam. Such a configuration provides advantages including: improvedtolerance to misalignment of the recording optics and simpler, lesscostly micro-holographic systems. Thus, a reflective element with littleor no curvature may be used in a micro-holographic system according toan aspect of the present invention. One surface of a data recording discmay be used as a reflective element (with or without a reflectivecoating).

For example, an injection-moldable thermoplastic media withlow-curvature features may be molded into the media surface and can bemetallized and used for generating the reflection as well as fortracking. According to an aspect of the present invention, athermoplastic media may be molded to incorporate slightly curvedelements into a disc, which may then be used for generating reflectionswith higher power density. These features may be well suited fortracking, like grooves on a DVD. Further, one or more elements may beused to correct the reflected light beam. For example, a curved mirrormay be used to generate a collimated light beam and a liquid crystalcell may be used to offset the path length difference generated by goingto different layers. Or, a holographic layer that acts like adiffractive element may be positioned near a surface of the medium, soas to provide correction to the light beam. An external mirror or thedisc surface may be used to generate the reflection.

According to an aspect of the present invention, data readout atdifferent layers may be different. Because the reflections havedifferent aberrations at different layers, the aberration may be usedfor layer indexing in a focusing process. Designs at the backside of thedisk may be used to provide for better control of a reflected light beamin order to increase effective grating strength. Multi-layer coatingsand/or surface structures (similar to display film structures) aresuitable for use. According to an aspect of the present invention, adesign that absorbs oblique incidence light beams and reflectsperpendicular light beams may also be used to both reduce noise andcontrol the orientation of the micro-holograms. Further, gratingstrength of micro-holograms need not be the same for different layers.Power scheduling may be used for recording at different layers.

According to an aspect of the present invention, recordingmicro-holograms using one focused light beam and one plane-wave lightbeam in a threshold material may be effected. While such a method mayutilize two input light beams, alignment requirements are less stringentthan conventional methodologies, while micro hologram orientation andstrength remain well controlled and uniform through layers. Readoutsignal may be better predicted as well.

Single-Bit Holography

Single bit micro-holography presents several advantages for optical datastorage over other holographic techniques. Referring now to FIG. 1,there is shown an exemplary configuration 100 for forming a hologramwithin a media using counter-propagating light beams. Therein,micro-holographic recording results from two counter-propagating lightbeams 110, 120 interfering to create fringes in a volume 140 of arecording medium 130. Interference may be achieved by focusing lightbeams 110, 120 at nearly-diffraction-limited diameters (such as around 1micrometer (μm) or smaller) at a target volume, e.g., desired location,within recording medium 140. Light beams 110, 120 may be focused using aconventional lens 115 for light beam 110 and lens 125 for light beam120. While simple lensing is shown, compound lens formats may of coursebe used.

FIG. 2 shows an alternative configuration 200 for forming a hologramwithin a hologram supporting media using counter-propagating lightbeams. In configuration 200, lens 125 has been replaced by a curvedmirror 220, such that a focused reflection 120 of light beam 110interferes with light beam 110 itself. Configurations 100, 200 requirehighly precise alignment of both lenses 115, 125 or of lens 115 andmirror 220 relative to each other. Accordingly, micro-holographicrecording systems employing such a configuration are limited to stable,vibration-free environments, such as those incorporating conventionalhigh-precision positioning stages.

According to an aspect of the present invention, a focused, slightlyfocused or unfocused reflected light beam (relative to acounter-propagating focused light beam) may be used for recording. FIG.3 shows an alternative configuration 300 to forming a hologram within amedia using counter-propagating light beams. Configuration 300 uses anunfocused counter-propagating reflection 310 of light beam 110 frommirror 320. In the illustrated embodiment, mirror 320 takes the form ofa substantially planar mirror.

FIG. 4 shows an alternative configuration 400 for forming a hologramwithin a media using counter-propagating light beams. Configuration 400uses a slightly-focused counter-propagating reflection 410 of light beam110 from mirror 420. The illustrated embodiment of configuration 400also includes optical path length correction element 425, that may takethe form of a liquid crystal cell, glass wedge, or wedge pair, forexample.

FIG. 5 shows another alternative configuration 500 for forming ahologram within a media using counter-propagating light beams. Similarto configuration 300 (FIG. 3), configuration 500 uses a substantiallyplanar reflecting surface. However, configuration 500 uses a portion 520of media 130 itself to provide reflection 510 of light beam 110. Portion520 may take the form of a reflective (such as a metal coated) rearsurface of media 130, a reflective layer within media 130 or one or moreholograms essentially forming a reflective surface in media 130, all byway of non-limiting example.

In configurations 300, 400 and 500, light beam 110 has a smaller spotsize and larger power density in a target volume or region than lightbeam 310, 410, 510, such that the micro-hologram dimensions will bedriven by the dimensions of the smaller spot size. A potential drawbackto the difference in power density between the two light beams is aresulting pedestal or DC component in the interference pattern. Such apedestal or DC component consumes a considerable portion of therecording capabilities (dynamic range) of material 130, where material130, exhibits a linear change of refractive index with experiencedexposure intensity.

FIG. 6 shows that experienced light intensity from counter-propagatinglight beams varies with position—thereby forming the interferencefringes. As is shown in FIG. 7, in a linearly responsive material, whererefractive index changes substantially linearly with experienced lightintensity relative to n_(o), the (relatively) unfocused light beam maythus consume dynamic range in a volume much larger than the targetvolume corresponding to the desired hologram, thereby decreasing thepossible reflectivity of other volumes and micro-holograms. Dynamicrange is also consumed throughout the depth of the media where thecounter-propagating light beams are at normal incidence as well (see,e.g., FIGS. 1 and 2).

According to an aspect of the present invention, such a consumption ofdynamic range in affected volumes other than the target volume duringhologram formation is mitigated by using a recording material exhibitinga non-linear response to experienced power density. In other words, anon-linear recording property exhibiting media is used in combinationwith a micro-holographic approach. The non-linear recording property ofthe material is used to facilitate recording that is non-linear withlight intensity (e.g. square, cubic, or of the threshold type), suchthat recording occurs substantially only above a certain lightintensity. Such a non-linear recording characteristic of the materialreduces or eliminates consumption of dynamic range in non-addressedvolumes, and facilitates reduction of dimensions of the micro-holograms,and thus target volumes.

FIGS. 10A-B and 11A-B illustrate recording characteristics of a linearrecording medium, while FIGS. 10C-D and 11C-D illustrate recordingcharacteristics of a non-linear recording medium of a threshold type.More specifically, FIGS. 10A-10D show that interfering two focused,counter-propagating light beams, as shown in FIGS. 1 and 2, produces amodulation of the light intensity, where position 0 (mid-way between−0.5 and 0.5) corresponds to the focal point along the medium thicknessof both focused light beams. In the case of a medium presenting linearrecording properties, a refractive index modulation like that shown inFIG. 10B will result—which follows the intensity profile shown in FIG.10A. While the refractive index modulation may ultimately maximize nearposition 0, it may be noted that it extends substantially over the fullthickness of the material and is not limited, for example, to theposition (abscissa) values in FIG. 10B—such that resultingmicro-holograms are not substantially contained within a particularvolume within the media, where multiple volumes are stacked one-uponanother. In a non-linear or threshold property exhibiting recordingmedium on the other-hand (a threshold condition being shown in FIG.10D), recording 1010 occurs substantially only in the volumes where athreshold condition 1020 is reached such that resulting micro-hologramsare substantially contained within a particular volume, where multiplevolumes are stacked one-upon another. FIG. 10D shows that themicro-hologram inducing fringes extend over approximately 3 μm. Similarcharacteristics are exhibited in the lateral dimensions of themicro-hologram as illustrated in FIGS. 11A-11D. As is demonstratedthereby, undesirable consumption of the dynamic range of untargetedvolumes of a media is mitigated by using a non-linear material of thethreshold type.

While a threshold type non-linear material is discussed for purposes ofexplanation, it should be understood that to a first-orderapproximation, the amplitude of the refractive index modulation varieslinearly with the light intensity in a linear responsive material (seeFIGS. 10A-10B, 11A-11B). Thus, even though a material having a recordingthreshold may prove particularly desirable, a material that exhibits anon-linear optical response to exposure in which the amplitude of therefractive index modulation varies, e.g., like a power larger than one(or a combination of powers) would significantly mitigate dynamic rangeconsumption in other affected volumes.

Returning again to the threshold type of non-linear material, andreferring again to FIGS. 10C-D and 11C-D, in such a case, athreshold-responsive media operates by experiencing an optically-inducedrefractive index change 1010 substantially only when the incident energydensity or power density 1015 is above a threshold 1020. Below threshold1020, the media experiences substantially no change in refractive index.One of the counter-propagating light beams, e.g., a reflected lightbeam, used for recording may be focused (FIGS. 1 and 2), slightlyfocused (FIG. 4) or even unfocused (FIGS. 3 and 5). Using such athreshold responsive material nonetheless has the affect of lesseningfocusing tolerance requirements. Another advantage of is that thereflective device may be incorporated into the media, such as a disc,similar to current surface technology optical storage devices, such asis illustrated in FIG. 5.

Referring now also to FIGS. 8 and 9, using smaller micro-holograms, asopposed to larger page-based holograms, provides improved systemtolerance to temperature fluctuations and angular misalignments. FIG. 8illustrates expected Bragg detuning of a hologram (∝1/L, where L is thehologram length) as a function of the difference between record and readtemperature. Reference 810 corresponds to expected performance of amicro-hologram, while reference 820 corresponds to expected performanceof a page-based hologram. FIG. 9 illustrates expected Bragg detuning ofa hologram (∝1/L, where L is the hologram length) as a function ofangular change. Reference 910 corresponds to expected performance of amicro-hologram, while reference 920 corresponds to expected performanceof a page-based hologram.

By way of non-limiting, further explanation only, an incoming light beamfocused at nearly-diffraction limited size may be reflected with aslight focusing or no focusing at all, such that the reflected lightbeam is unfocused (or slightly focused) relative to thecounter-propagating, focused incoming light beam. The reflective elementmay be on a disc surface, and may take the form of a flat mirror, or aslightly curved mirror, for example. If some misalignment occurs betweenthe focused light beam and reflection, the interference pattern will bedriven by the location of the focused light beam where the reflectedlight beam has a relatively large curvature of its phase front. Thelarge curvature produces small power density variation when the focusedspot moves relative to the reflected light beam.

NON-LINEARLY RESPONSIVE MATERIAL EXAMPLE 1

Photopolymers have been proposed as a media candidate for holographicstorage systems. Photopolymer based media have reasonable refractiveindex changes and sensitivities recorded in a gel-like state sandwichedbetween glass substrates. However, it is desirable to provide asimplified structure, such as a molded disc. Further, photopolymersystems are sensitive to environmental conditions, i.e., ambientlighting, and often require special handling prior to, during and evensometimes after the recording process. It is desirable to eliminatethese drawbacks as well.

According to an aspect of the present invention, a polymer phase-changematerial in which refractive index modulations are induced via exposureto a light beam is used as a holographic data storage medium. In oneembodiment, the detectable change in refractive index results fromthermally inducing localized changes between amorphous and crystallinecomponents of the material. This provides for potentially largerefractive index modulations induced using small energies. Such amaterial may also provide for a threshold condition, in which opticalexposure energies below a threshold have little or substantially noimpact on the refractive index of the material, while optical exposureenergies above the threshold cause detectable refractive index changes.

More particularly, a phase-change induce-able polymer material canprovide large refractive index changes (Δn>0.01), with good sensitivity(S>500 or more cm/J), in an injection-moldable, environmentally-stable,thermoplastic substrate. Additionally, such a material also offers thepotential to use a substantially threshold-responsive recordingprocess—enabling a same wavelength laser to be used for both reading andwriting, while preventing ambient light exposure from substantiallydegrading stored data. In one embodiment, the detectable refractiveindex change corresponds to the index difference between the amorphousand crystalline states of one of the components of a copolymerthermoplastic substrate. Such a substrate can be prepared by elevatingthe copolymer above the melting temperature (Tm) and rapidly cooling, orquenching, the material to induce the previously crystalline componentsof the material to cool in an amorphous state.

Referring now also to FIGS. 14A and 14B, light beams are interferedwithin target volumes of the material to locally heat sub-volumesthereof corresponding to the interference fringes, as a result of energyabsorption thereat. Once the local temperature is raised above thecritical temperature, for example the glass transition temperature (Tg)(FIG. 14A), the crystalline components of the material melt andsubsequently cool into an amorphous state, resulting in a refractiveindex difference relative to the other crystalline state volumes in thematerial. The critical temperature may alternatively be around themelting temperature (Tm) of the nano-domain component material.Regardless, if the energy of the incident light beam is not sufficientto elevate the temperature of the material above the criticaltemperature, substantially no change takes place. This is shown in FIG.14B, where an optical fluence above a critical value F_(crit) causes aphase change resulting in the writing of a hologram, and an opticalfluence less than the critical value F_(crit) causes substantially nosuch change—and is thus suitable for reading recorded holograms, andhence recovering recorded data.

For non-limiting purposes of further explanation, the critical value isgiven by F_(CRIT)=L×ρ×c_(p)×ΔT, where L is the length, or depth, of amicro-hologram, ρ is the material density, c_(p) is the specific heat ofthe material, and ΔT is the experienced temperature change (i.e.,T_(g)-T₀, where T_(g) is the the glass transition temperature and T₀ isthe ambient temperature of the material). As an example, where apolycarbonate having a density of 1.2 g/cm³ and a specific heat of 1.2J/(K·g) is used, the length of the micro-hologram is 5×10⁴ cm, and thetemperature change is 125° C. (K), F_(CRIT)=90 mj/cm²translated toenergy terms, the energy (E_(CRIT)) needed to reach the critical fluenceF_(CRIT) is${E_{CRIT} = {{F_{CRIT} \times A} = {F_{CRIT} \times \frac{\pi\quad w_{o}^{2}}{2}}}},$where A is the transverse area of the hologram and w_(o) is the lightbeam waist. The energy at focus, E_(F), needed to provide E_(CRIT) is${E_{F} = \frac{E_{CRIT}}{( {1 - {\mathbb{e}}^{{- \alpha}\quad L}} )}},$where e^(αL) is the transmission, α=α₀+α_(NL)F, α₀ is the linearabsorption of the material, α_(NL) is the non-linear absorption of thematerial, F is the maximum incidence optical fluence, and L is thelength of the micro-hologram. The incident energy, E_(IN), delivered tothe material to provide needed energy at focus, E_(F), is${E_{IN} = \frac{E_{CRIT}}{( {1 - {\mathbb{e}}^{{- \alpha}\quad L}} ){\mathbb{e}}^{{- \alpha}\quad{D/2}}}},$where e^(−αL) is the transmission, α=α₀+α_(NL)F, α₀ is the linearabsorption of the material, α_(NL) is the non-linear absorption of thematerial, F is the the maximum incidence optical fluence, L is thelength of the micro-hologram, and D is the depth (or length) of thematerial (e.g., the thickness of the media disc). Referring now also toFIGS. 15A-15C, assuming a light beam waste, w_(o), of 0.6×10⁻⁴ cm, thetransverse area of the hologram, A, is 5.65×10⁻⁹ cm². Still assuming adepth of the micro-hologram, L, to be 5×10⁻⁴ cm, and the depth of thematerial D (e.g., entire media disc) to be 1 mm, a predicted relationbetween incident energy, E_(IN), and α is shown in FIG. 15A. Furtherassuming a material linear absorption, α₀, of 0.018 1/cm, and a materialnon-linear absorption, α_(NL), of 1000 cm/J (and still a material lengthof 0.1 cm), a predicted relation between transmission and fluence isshown in FIG. 15B. Using these same assumptions, predicted relationsbetween light beam waist and distance, and normalized absorption anddistance are shown in FIG. 15C.

Consistently, and as is shown in FIGS. 16A and 16B, it is expected thatcounter-propagating light beam exposure of such a copolymer materialmedia will write micro-holograms in the form of fixed index modulationscorresponding to the counter-propagating light beam interference fringesdue to the formation or destruction of nano-domains of crystallinepolymer thereat. That is, the phase change/separation mechanismgenerates a refractive index modulation based on the formation ordestruction of crystalline nano-domains that are substantially smallerthan the wavelength of the light being used. The values of FIG. 16B arepredicted using two counter-propagating beams each having an incident,single beam power (P1=P2) of 75 mW, α=20 cm⁻¹ and an exposure time (τ)of 1 ms. A predicted resulting refractive index change (Δn=0.4) thatforms the micro-hologram is shown in FIG. 16C. As can be seen therein, amicro-hologram embodied as a series of refractive index changescorresponding to interference fringes of counter-propagating light beamsoccurs substantially only where a localized heating exceeds a thresholdcondition (e.g., the temperature exceeds 150° C.), such that a thresholdrecording condition results.

Suitable polymers for use, include, by way of non-limiting example,homopolymers displaying partial crystallinity, blends of homopolymerscomposed of amorphous and crystalline polymers, and a variety ofcopolymer compositions including random and block copolymers, as well asblends of copolymers with or without homopolymers. Such a material issuitable for storing holograms on the order of 3 micrometers (microns)deep, by way of non-limiting example only. The linear absorption of thematerial may be high, rendering the material opaque and limiting thesensitivity.

A thermally induced reaction responsive to an optically absorbing dye iswell suited for separating the index change mechanism from thephoto-reactive mechanism, enabling potentially large sensitivities.According to an aspect of the present invention, the thermally inducedprocess may provide the non-linear responsive mechanism for theoptically induced refractive index change. This mechanism, or thresholdcondition, enables optical beams of a same wavelength to be used at lowand high powers for data reading and recording, respectively. Thischaracteristic also prevents ambient light from substantially degradingthe stored data. Dyes with a reverse saturable absorption (RSA)property, in which the absorption is a function of the fluence andincreases with increasing fluence, are useful. As a consequence,absorption is highest at the light beam(s) focus, which means backgroundlinear absorption is small, ultimately yielding a material that isnearly transparent. Examples. of such dyes include porphyrins andphthalocyanines, by way of non-limiting example only.

Further, amorphous/crystalline copolymers are well suited to provide thedesired properties in an injection-moldable thermoplastic substrate,such as a disc. The use of a thermoplastic enables data to be recordedin a stable substrate without significant post-processing requirements,such that the refractive index change, sensitivity, stability, and“fixing” are provided by the single co-polymer material itself. And,index modulations larger than conventional photopolymers may be possiblevia selection of copolymer components. The sensitivity of the materialmay depend on the optical absorption properties of dye(s) used. In thecase of known reverse saturable absorption dyes, sensitivities as highas 2-3 times conventional holographic photopolymers are achievable. Thethreshold condition also provides the ability to read and write data ata same wavelength with little or no post-processing required after thedata is recorded. This is in contrast to photopolymers, which typicallyrequire total substrate exposure after recording of data to bring thesystem to a full cure. Finally, the copolymer substrate may be in athermoplastic state, as opposed to the gel-like state of photopolymers,prior to data recording. This advantageously simplifies the physicalstructure of the media as compared to photopolymers, as thermoplasticstate material may be injection molded itself and need not be containedwithin a container or carrier, for example.

Thus, according to an aspect of the present invention,amorphous/crystalline copolymers may be used to support opticallyinduced phase changes and resultant index modulations. Linear absorbingdyes may be used in combination with amorphous/crystalline phase changematerials to convert optical energy to temperature increases. Reversesaturable absorption dye(s) may be used to efficiently generatetemperature increases. Optical activation may be separated from indexchange inducement via the dyes and phase change/separation materialsenabling a threshold condition to index change.

By way of further explanation, in certain block copolymer compositions,the individual polymers phase separate spontaneously into regularlyordered domain structures that do not grow macroscopically like polymerblends, because of the nature of the copolymer. This phenomena isdiscussed by Sakurai, TRIP vol. 3, 1995, page 90 et. seq. The individualpolymers making up the copolymer can display amorphous and/orcrystalline behavior depending on temperature. The weight ratio of theindividual polymers may tend to dictate whether the micro-phases thatseparate form spheres, cylinders or lamellae. A copolymer system inwhich both phases are amorphous upon a brief (or extended) heating abovethe glass transition temperature (Tg) and melting temperature (Tm) ofthe individual blocks may be used. Upon cooling to low temperatures, oneof the phases crystallizes, while maintaining the shapes of the originalmicro-phases. An example of this phenomenon is illustrated inpoly(ethylene oxide)/polystyrene block copolymers, as reported by Hunget al., in Macromolecules, 34, 2001, page 6649 et seq. According to anaspect of the present invention, poly(ethylene oxide)/polystyrene blockcopolymers may be used in a 75%/25% ratio, for example.

For example, a photo-chemically and thermally stable dye, such as aphthalocyanine dye, like Copper Phthalocyanine, Lead Phthalocyanine,Zinc Phthalocyanine, Indium Phthafocyanine, Indium tetra-butylPhthalocyanine, Gallium Phthalocyanine, Cobalt Phthalocyanine, PlatinumPhthalocyanine, Nickel Phthalocyanine,tetra-4-sulfonatophenylporphyrinato-copper(II) ortetra-4-sulfonatophenylporphyrinato-zinc(II) can be added to such acopolymer and injection molded into a 120 mm diameter disc. The moldingraises the temperature of the copolymer above the glass transitiontemperature (Tg) of the polystyrene and the melting temperature (Tm) ofthe poly(ethylene oxide), thus producing an amorphous material withmicro-phase separations. Cooling, e.g., quenching, of the disc to about−30° C. causes the poly(ethylene oxide) phase to crystallize throughoutthe material. Where the domain sizes of the crystalline regions aresufficiently small, such as less than one hundred nanometers (e.g.,<100nm), light will not be scattered by the media, and the media will remaintransparent even in thick substrates. Data may be recorded into thematerial by interfering 2 laser beams (or a light beam and a reflectionthereof) at specific regions, e.g., in target volumes, of the disc.

Upon exposure to one or more recording light beams (e.g., high powerlaser beams), the dye absorbs the intense light at the interferencefringes, momentarily raising the temperature in the corresponding volumeor region of the disc to a point above the melting temperature (Tm) ofthe poly(ethylene oxide) phase. This causes that region to becomesubstantially amorphous, producing a different refractive index than thecrystalline domains in the surrounding material. Subsequent exposure tolow energy laser beams for the purpose of reading the recordedmicro-holograms and recovering corresponding data as micro-hologramreflections does not cause any substantial change in the material, wherelaser powers that do not heat the polymer above the Tg or Tm of theindividual polymers are used. Thus, a non-linear optically responsive,such as a threshold responsive, holographic data storage media may beprovided that is substantially stable for long periods of time and overa number of readings.

While spheres, cylinders and lamellas are common structures, otherpermutations can form and work equally well. A variety of blockcopolymers, including polycarbonate/polyester block copolymers, mayalternatively be used and allow for different forming temperatures ofthe crystalline domains, as well as the temperature at which they aredestroyed. Where the dye used to absorb the radiation and produce heattakes the form of a reverse saturable absorber, good control inpinpointing where the heating takes place may result. Lateral extensionof the micro-holograms may be significantly smaller than the diameter ofthe waist of the focused laser beam(s). Limiting or eliminatingconsumption of dynamic range of the recording material outside of therecorded micro-holograms, hence increasing reflectivity of eachmicro-hologram and therefore data storage capacity, may thus be realizedthrough the use of a non-linear recording medium according to an aspectof the present invention.

A threshold material can also present the additional benefit of beingmore sensitive to recording than a linear material. This advantage maytranslate into higher achievable recording data rates for amicro-holographic system. Further, a step-wise refractive indexmodulation resulting from a threshold characteristic of the media mayserve to produce micro-holograms less reflective than when using linearmaterials. However, reflectivity may remain sufficiently high for datastorage applications. Referring now also to FIG. 12, it is expected thatreflectivity will increase with increasing refractive index modulation.It is also expected that thermal diffusion should not present undueproblems. Thermal diffusion during hologram formation has also beenconsidered, and the temperature pattern is expected to follow theinterference fringes of the counter-propagating light beams, i.e., theexposure pattern. To maintain the fringes in the index pattern, thermaldiffusion may be substantially limited to the region between the fringesreaching the phase change temperature. Curve 1210 in FIG. 12 correspondsto a linearly responsive material, and curve 1220 in FIG. 12 correspondsto a threshold responsive material. Referring now also to FIGS. 13A and13B, there are shown expected temperature elevation profiles as afunction of position. Accordingly, it is expected that thermal leakagefrom a target volume to surrounding volumes should not raise thesurrounding volumes to the threshold temperature 1020.

NON-LINEAR MATERIAL EXAMPLE 2

According to another configuration, organic dyes in polymer matrices maybe used to support refractive index changes (Δn) to effect holographicdata storage, where the organic dyes have large resonant enhancedrefractive indices relative to the polymer matrix. In such a case,bleaching of the dyes in specific regions, or target volumes, may beused to produce the refractive index gradient for holography. Data maybe written by interfering light beams within the media to bleachspecific areas. However, where interfering light passes through theentire media, (even though only specific areas are to be bleached) and alinear response to the bleaching radiation exists, (even though thelight beam intensity is highest in the focused areas, and produces themost bleaching thereat) relatively low levels of the dye are expected tobe bleached throughout the impinged media. Thus, after data is writteninto multiple levels, an undesirable additional bleaching of is expectedto occur in a linear recording media. This may ultimately limit thenumber of layers of data that can be written into the media, whichin-turn limits overall storage capacity for the linear recording media.

Another concern arises from the recognition that a recording mediumneeds to have a high quantum efficiency (QE) in order to have a usefulsensitivity for commercial applications. QE refers to the percentage ofphotons hitting a photo reactive element that will produce anelectron-hole pair and is a measure of the device's sensitivity.Materials with high QEs are typically subject to rapid bleaching ofstored holograms, and thus data, even when using a low power readinglaser. Consistently, data can only be read a limited number of timesbefore the data essentially become un-readable in a linearly responsivemedium.

According to an aspect of the present invention, a non-linear opticallyresponsive medium is used to address these shortcomings. Again, amaterial solution based on thermoplastics, instead of photopolymers, maybe used in a holographic system for providing data storage andretrieval. This may prove advantageous in terms of processes, handlingand storage, as well as compatibility with a variety of holographictechniques.

By way of further explanation, narrow band absorbing dyes inthermoplastic materials may be used for holographic optical datastorage. It is believed that rigid polymer networks retard QuantumEfficiencies (QE) for certain photochemical reactions. Thus, accordingto an aspect of the present invention, localized heating of a polymernetwork, such as to temperatures near or above the Tg of thethermoplastic, are useful for increasing the localized QE of thematerial, such as by a factor >100. This improvement directly enhancesthe sensitivity of the material in a manner useful for holographicoptical data storage. Further, it provides a gating process, or athreshold process, in which dye molecules in discrete molten regions ofthe media undergo photochemical reactions faster than in the surroundingamorphous material—in turn facilitating writing on many virtual layersof a media without significantly affecting other layers. In other words,it enables reading and writing without deleteriously causing significantbleaching of other volumes.

Referring now to FIGS. 17A-17C, ortho-nitrostilbenes (o-nitrostilbenes)containing polymer matrices may be used for holographic data storage.The photochemical reaction that causes bleaching of ortho-nitrostilbenesis well known, and discussed for example in Splitter and Calvin, JOC,1955, vol. 20 and pages 1086-1115. McCulloch later used this class ofcompound for producing waveguides in a thin film application bybleaching the dye to form cladding material (see, Macromolecules, 1994,vol. 27, pages 1697-1702. McCulloch reported a QE of a particularo-nitrostilbene to be 0.000404 in a Polymethylmethacrylate (PMMA)matrix. However, he noted the same dye in a dilute hexane solution had aQE of 0.11 at the same bleaching wavelength. McCulloch furtherspeculated this difference was due to hypsochromic shift in the lambdamax in going from thin polymer films to hexane solutions. It may berelated to a mobility effect, since the stable conformation of theo-nitrostilbene in the rigid polymer may not be aligned properly due tothe initial pericyclic reaction. FIG. 17A illustrates data indicative ofbleaching with a 100 mW 532 nm laser at 25° C. and at 160° C.Enhancement may be due to increased mobility or simply faster reactionkinetics due to the higher temperature, or a combination of both.Consistent with FIG. 17A, FIG. 17B shows an enhanced QE of the discussedmatrix is expected at above around 65° C. Thus, in one embodimento-nitrostilbene dyes are used in combination with polycarbonate matricesto provide performance comparable to PMMA materials, though slightlyhigher QEs may be possible.

It should be understood, however, that the present invention is not tobe restricted to this class of dyes. Rather, the present inventioncontemplates the use of any photoactive dye material having asufficiently low QE in a solid polymer matrix at or near roomtemperature and that displays an increase in QE, such as an exponentialincrease in QE, upon heating. This provides for a non-linear recordingmechanism. It should be understood that the heating need not raise thetemperature above glass transition temperature (Tg) or it may raise itwell above Tg, as long as the QE becomes significantly enhanced. The QEof such a photoactive dye may be enhanced within specific regions of apolymer matrix that contains a substantially uniform distribution of thedye. In the case of a polycarbonate matrix, by heating the polycarbonatematrix containing the photoactive dye above the Tg thereof, an increasein the bleaching rate may be achieved. The increase of the bleachingrate may be on the order of >100 times.

Optionally, in addition to a photo reactive dye being added to apolycarbonate matrix like o-nitrostilbene, a second thermally and photochemically stable dye may also be added to the matrix to function as alight absorber, to produce localized heating at the interference fringesat the focus of counter-propagating laser beams. Dye concentrations,laser power and time at the focusing point may be used to adjust theexpected temperature to the desired range near or above the Tg of thematrix, for example. In such an embodiment, the first and secondwavelengths of light for photo bleaching are simultaneously focused inroughly the same region of the matrix. Since the sensitivity in theheated region of the material is expected greater, e.g., on the order of100 times greater, than surrounding cool rigid polymer regions (see,FIG. 17A), information can be quickly recorded in a target, heatedvolume using a relatively low power light beam having a significantlyless bleaching affect on the surrounding regions. Thus, previouslyrecorded regions or regions that have not yet had data recordedexperience minimal bleaching, thereby mitigating undesired dynamic rangeconsumption thereat and permitting more layers of data to be written inthe media as a whole. Also, by reading at relatively low power with thelaser wavelength used to heat the specific region for writing,inadvertent dye bleaching during readout is also mitigated against.Alternatively, a single wavelength, or range of wavelengths, of lightmay be used for heating and bleaching, such that only one wavelength oflight (or range of wavelengths) is used instead of two differentwavelengths.

Although a variety of dyes are suitable for acting as thermally andphoto-chemically stable dyes for localized heating purposes, dyes thatbehave non-linearly may prove particularly well suited. One such classof dyes, known as Reverse Saturable Absorbers (RSA), also known asexcited state absorbers, is particularly attractive. These include avariety of metallophthalocyanines and fullerene dyes that typically havea very weak absorption in a portion of the spectra well separated fromother strong absorptions of the dye, but nonetheless form strongtransient triplet-triplet absorption when the intensity of the lightsurpasses a threshold level. Data corresponding to a non-limitingexample using extended dimethylamino dinitrostilbene is shown in FIG.17C. Consistently therewith, it is expected that once an intensity oflight at interference fringes of counter-propagating light beams in amedium incorporating dimethylamino dinitrostilbene surpasses thethreshold level, the dye absorbs strongly at a focused point and canquickly heat the corresponding volumes of the material to hightemperatures. Thus, according to an aspect of the present invention, athermal gating event is used to enable a relatively low energy to writedata into a target volume of a media (thus exhibiting increasedsensitivity), while minimizing unwanted exposure induced reactions inother volumes of the media.

Tracking and Focusing

In one embodiment, micro-holograms are stored in a volumetric mediumalong radially extending spiral tracks in a plurality of verticallystacked layers where the media is in the form of a disc that spins (see,e.g., FIGS. 28 and 30). An optical system focuses a light beam intoparticular target volumes in the media, to detect the presence orabsence of a micro-hologram thereat, in order to recover or read out thepreviously stored data or to generate interference fringes thereat togenerate a micro-hologram. Thus, it is important that target volumes beaccurately targeted for data writing and recovery light beamillumination.

In one embodiment, the spatial characteristics of reflections of animpinging light beam are used to aid accurate targeting of selectedvolumes of the micro-hologram array containing media. If a targetvolume, e.g., micro-hologram, is out of focus or off track, thereflected image differs from a reflection from a micro-hologram that isin-focus and on-track in a predictable manner. This can in-turn bemonitored and used to control actuators to accurately target specificvolumes. For example, the size of reflections from micro-holograms outof focus varies from those of micro-holograms in focus. Further,reflections from misaligned micro-holograms are elongated as compared toreflections from properly aligned micro-holograms, e.g., are moreelliptical in nature.

By way of further explanation, in the above-discussed material systems,(different from conventional CD and DVD technologies) a non-metalizedlayer is used to reflect an incident reading light beam. As shown inFIG. 18, micro-hologram 1810 contained in media 1820 reflects readinglight beam 1830 to a ring detector 1840 positioned around one or moreoptical elements (e.g., lens) 1850. Optical element 1850 focuses lightbeam 1830 into a target volume corresponding to micro-hologram 1810—suchthat micro-hologram 1810 generates a reflection that is incident onoptical element 1850 and ring detector 1840. In the illustratedembodiment, optical element 1850 communicates the reflection to a datarecovery detector (not shown). It should be understood that while only asingle micro-hologram 1810 is illustrated, in actuality media 1820 isexpected to contain an array of micro-holograms positioned at variouspositions (e.g., X, Y coordinates or along tracks) and in many layers(e.g., Z coordinates or depth planes or pseudo-planes). Usingactuator(s), optical element 1850 may be selectively targeted todifferent target volumes corresponding to select ones of themicro-holograms.

If micro-hologram 1810 is at the focus of the reading light beam 1830,the reading laser beam 1830 gets reflected, thereby generating areflected signal at optical element 1850, which is communicated to adata recovery detector. The data recovery detector may take the form ofa photo-diode positioned to detect light beam 1830 reflections, forexample. If no micro-hologram 1810 is present at the focus, nocorresponding signal is generated by the data recovery detector. In adigital data system, a detected signal may be interpreted as a “1” andthe absence of a detected signal as a “0”, or vice-a-versa. Referringnow also to FIGS. 19A-19C, there is shown simulated reflection datacorresponding to an on-focus, on-track circular micro-hologram, using areading light beam having an incident wavelength of 0.5 μm, a laser spotsize of D/2=0.5 μm, a left circular polarization, a con-focal light beamparameter: z/2=2.5 μm, and a far field half diffraction angle ofθ/2=11.55° (field) or θ/2=8.17° (power).

Referring now also to FIG. 20, in order for a reading laser beam to bereflected by a micro-hologram correctly, the laser beam should becorrectly focused and laterally centered on the micro-hologram. In FIG.20, an incident light beam is seen to have wave-fronts 2010 that arenormal to the propagation optical axis 2020 in the central portion 2030thereof. A micro-hologram substantially only reflects the light of thosewave vectors (i.e., k vectors) that match a certain direction. A focusedGaussian light beam, such as that shown in FIG. 20, is the overlap ofmany wavelets with various wave vector. The maximum angle of the wavevector is determined by the numerical aperture of the focusing objectivelensing. Accordingly, not all wave vectors are reflected by themicro-hologram—such that a micro-hologram acts like a filter that onlyreflects incident light with certain wave vectors. When away from focus,only the central portion of the incident light overlaps with themicro-hologram. So, only the central portion gets reflected. In thisscenario, changes in the reflection efficiency decrease.

When the focused light beam is not properly aligned with amicro-hologram in a track, the wave vectors along the direction verticalto the track do not have as strong a reflection in the direction alongthe track. In such a case, the light beam is elongated in the directionvertical to the track in the near field, while the light beam issqueezed in this direction in the far field. Accordingly, separatetracking holograms may be provided.

FIGS. 21A-21C show near-field distributions (z=−2 μm) corresponding tothe simulation of the circular micro-hologram of FIGS. 19A-19C. FIG. 21Aillustrates a data recovery light beam being launched at x=y=0 andz=0.01 into a media. FIG. 21B illustrates an off-track conditionreflection caused by a shift of x=0.5. FIG. 21C illustrates an out of-or off-focus condition reflection caused by a shift of z=1.01. Thus, inan out of focus condition light beam efficiency decreases, while in anoff track condition the reflection is spatially distorted. Referring nowalso to FIGS. 22A-22C, there are shown far-field distributionscorresponding to the near-field distributions of FIGS. 21A-21C,respectively. FIG. 22A shows a data recovery light beam being launchedat x=y=0 and z=0.01 into a media provides analogous far-field divergenceangles (full) in the X and Y directions, in the illustrated case 11.880in both X- and Y-directions. FIG. 22B shows an off-track conditionreflection caused by a shift of x=0.5 results in different far-fielddistribution angles in the X and Y, in the illustrated case 4.6° in theX-direction and 6.60 in the Y-direction. Finally, FIG. 22C shows an outof- or off-focus condition reflection caused by a shift of z=1.01results in analogous far-field divergence angles (full) in the X and Ydirections, in the illustrated case 9.94° in both X- and Y-directions.Thus, micro-holograms act as k-space filters, such that the far fieldspot will be elliptical in an off-track condition, and the far fieldspot will be smaller with an out of focus condition.

It should be understood that the micro-holograms need not be circular.For example, oblong micro-holograms may be used. Referring now also toFIGS. 23A-23C, there is shown a simulation corresponding to an on-focus,on-track oblong micro-hologram, using a reading light beam having anincident wavelength of 0.5 μm, a laser spot size of D/2=0.5 μm, a leftcircular polarization, a Rayleigh range of z/2=2.5 μm, and a far fieldhalf diffraction angle of θ/2=11.55° (field) or θ/2=8.17°(power)—analogous to the simulation of FIGS. 19A-19C. FIGS. 24A-24C shownear-field distributions (z=−2 μm) corresponding to the simulation ofthe oblong micro-hologram of FIGS. 23A-23C. FIG. 24A illustrates a datarecovery light beam being launched at x=y=0 and z=0.01 into a media.FIG. 24B illustrates an off-track condition reflection caused by a shiftof x=0.5. FIG. 24C illustrates an out of- or off-focus conditionreflection caused by a shift of z=1.01. Thus, in an out of focuscondition, light beam efficiency decreases, while in an off trackcondition the reflection is spatially distorted. Referring now also toFIGS. 25A-25C, there are shown far-field distributions corresponding tothe near-field distributions of FIGS. 24A-24C, respectively. FIG. 25Ashows a data recovery light beam being launched at x=y=0 and z=0.01 intoa media provides far-field divergence depending upon the oblong-ness ofthe micro-hologram, in the illustrated case 8.23° in the X-direction and6.170 in the Y-direction. FIG. 25B shows an off-track conditionreflection caused by a shift of x=0.5 results in different far-fielddistribution angles in the X and Y, in the illustrated case 4.330 in theX-direction and 5.080 in the Y-direction. Finally, FIG. 25C shows an outof- or off-focus condition reflection caused by a shift of z=1.01results in different far-field divergence angles (full) in the X and Ydirections, in the illustrated case 5.880 in the X-direction and 5.000in the Y-direction.

Thus, oblong micro-holograms also act as k-space filters, and that whileoblong micro-holograms result in elliptical far-field spot spatialprofiles, in an off track condition the elongated direction may differ,and the far field spot will be smaller with an out of focus condition.

The present invention will be further discussed as it relates tocircular micro-holograms for non-limiting purposes of explanation only.The light beam shape variation in the off track direction, as well aslight beam spatial intensity, may be determined using a quadropoledetector, such as that shown in FIG. 26. Thus, in one embodiment, thespatial profile of micro-holograms reflections are used to determinewhether a reading light beam is in focus and/or on track. This signalmay also serve to separate the two light beam focusing scenarios, out offocus and out of track, and provide a feedback signal to a drive servoto correct the position the laser optics head, for example. For example,one or more detectors that convert micro-hologram reflections intoelectrical signals can be used to detect changes in the reflected imageof the micro-holograms—and hence be used to provide focus and trackingfeedback for optical element positioning actuators. A variety ofphotodetectors may be used to detect the micro-hologram reflections. Asan example, one or more photodiodes may be used to detect reflectionsfrom micro-holograms in a conventional manner. The manufacturing and useof photodiodes are well known to those possessing an ordinary skill inthe pertinent arts. The information provided by these detectors is usedto perform real-time control of actuators in the optical system in orderto maintain focus and stay on the correct data track.

Such a servo control system may thus address primarily two scenariosthat can occur for laser beam out of focus condition: the first is whenthe laser beam is not focused onto the correct layer, and the second iswhen the laser beam is laterally misaligned from the micro-hologram tobe read; while also being configured to optimize tracking and focusperformance in the presence of noise sources. Estimation techniques,such as Kalman filters, can be used to deduce an optimal estimate ofpast, present, or future states of the system in order to reduce thereal-time errors and reduce read and write errors.

FIGS. 26A-26D show a detector configuration or array (FIG. 26A) andvarious detected conditions (FIGS. 26B-26D) for determining whether thesystem is in focus or on track. In one embodiment, a four quadrantdetector array 2600 may be used to determine if the optical system isout of focus or off-track. Each quadrant detector 2600A, 2600B, 2600C,2600D of detector array 2600 generates a voltage that is proportional tothe amount of energy reflected onto it. Detector array 2600 incorporatesan array of photodiodes that each correspond to one of the quadrants,such as in the form of a quadrapole detector, for example. In theillustrated embodiment detector array 2600 is responsive to opticalenergy propagating over an area greater than the focusing optics (e.g.,lens 2620) used to relay (e.g., focus) light beams in and reflectionsout of the volumetric storage media. For example, quadrapole detector2600 may be positioned behind objective lensing used to impinge andreceive reflections from a target volume, to detect light beam shapevariations. In the case of a circular micro-hologram, if the detectedlight beam shape is elliptical, it may be inferred that the light beamis off track, such that the off-track direction is the elliptical lightbeam short axis. If the detected light beam is smaller than expected(with smaller numerical aperture), but the variation is symmetric innature, it may be inferred the light beam is out of focus. Thesedetected changes in the spatial profile of the reflected read light beamfrom volumetric media are used as feedback for a drive focusing and/ortracking control. Optionally, a smaller lens array may be used aroundthe objective lensing to focus the distorted reflected signal. Further,changes in the angle of propagation of the reflected light beam are alsouseful as an indication of the direction of misalignment.

The total amount of signal generated by quadrant ring detectors2600A-2600D is represented by α. If the system is in focus, as is shownin FIG. 26B, the focused spot will be circular, of minimum size andproduce the least amount of signal α_(min). Where α>α_(min), as is shownin FIG. 26C, the light beam spot may be determined to be out of focus.Lens 2620 may be positioned in the center of detector array 2600 to passand focus a reading light beam on the micro-holograms. Conventionalfeedback control mechanisms that minimize a may be used to maintainfocus of the micro-hologram. Referring now also to FIG. 26D, anasymmetrical pattern is detected if the sensor head is moving off-track.When on-track, all four quadrant detectors 2600A, 2600B, 2600C, 2600Dreceive equal energy, such that β=(1800B+1800D)−(1800A+1800C)=0. Thus, acondition β≠0 indicates an off-track condition. By way of furtherexample, the reflected signal becomes elongated if the sensor head isoff track and variable β (the difference between opposite quadrants)becomes more positive or negative. Conventional feedback controlmechanisms may be used in combination with a tracking servo to reducethe tracking error by minimizing the absolute value of β. In oneembodiment, a time reference can be established so a and β are sampledat suitable times. A phase locked loop (PLL) may be used to establishthis reference and form a sampled tracking and focusing control system.Information from the rotational rate of the disc and the current readhead location may also be used to generate a master time reference, T,for the system.

Error sources, such as an off-center disk, disc warping and/or missingdata can be compensated for Kalman filters may be used to account forerror sources, and predict a future path of recorded micro-hologramsbased on past information. Normal progression of the spiral pathtrajectory can also be estimated and forwarded to the tracking servos.This information is useful for enhancing the performance of the trackingand focusing servos, and reducing tracking and focusing servo error.FIG. 27, shows a block diagram of a servo system 2700 suitable forimplementing focus and tracking control. System 2700 including focus andtrack path estimators 2710, 2720, that in one embodiment take the formof conventional Kalman filters. Focus path Kalman filter 2720 uses aservo timing pulse (τ), a rotational speed of the media, a focus errorvalue (ε) (the difference between the desired track path and the actualtrack path), and a current stylus (e.g., read head) location to providean estimated focus trajectory as the media rotates. Track path Kalmanfilter 2720 uses the servo timing pulse (τ), a rotational speed of themedia, track error value (ε), and the current stylus location to providean estimated track trajectory. System 2700 also includes a hologramdetecting, edge detecting, servo timing pulse (τ) providing phase lockedloop (PLL) 2730, that provides servo timing pulse (τ) responsively todetected total signal α, a motor timing signal that is directly relatedto the speed of the motor and the current stylus location. Conventionalconditioning circuitry 2740, e.g., incorporating differentialamplifiers, provides the total signal α, as well as the afore-discussedsignal β, responsively to quadrant detectors 2600A, 2600B, 2600C, 2600D(FIG. 26A).

A focus servo 2750 controls focus actuator(s) 2760 responsively to theestimated focus trajectory from focus path Kalman filter 2710, as wellas servo timing pulse (τ), total signal a, and a layer seek command fromconventional layer and track seek logic (not shown). A tracking servo2770 controls a tracking actuator(s) 2780 responsively to the estimatedtrack trajectory from track path Kalman filter 2720, as well as servotiming pulse (τ), signal β, and a track seek command from theconventional layer and track seek logic (not shown). In essence,actuators 2760, 2780 position and focus a reading and/or writing lightbeam into a target volume of the head in the media responsively tocorresponding layer and track seek commands from conventional layer andtrack seek logic (not shown).

Thus, there is disclosed a method of focusing and trackingmicro-holograms in a spatial storage medium. A master system timingreference is generated for a sampled tracking and focusing. Errorsignals are generated based on micro-holograms reflection asymmetryresulting from an off-track condition and/or expansion resulting from anout of focus condition. Kalman filters are used to estimate and correctfor tracking path errors in a tracking control servo formicro-holograms. Kalman filters may are used to correct for focus patherrors in a focus control servo for micro-holograms. The servo controlcan be used if the data are based on different layers or changes betweenlayers.

It should be understood that the tracking and focusing systems andmethods described herein are not limited to volumetric storage systemsand methods using non-linear and/or threshold responsive materials, butinstead have broad applicability to volumetric storage systems andmethods in general, including those using linearly responsive materials,such as that described U.S. Patent Publication 20050136333, the entiredisclosure of which is hereby incorporated by reference.

Formatting for Rotatable Volumetric Storage Disc Using Data IndicativeMicro-Holograms For Tracking

As set forth herein, micro-holograms can be stored in a rotating discusing multiple vertical layers and along a spiral track on each layer.The format of the data storage media may have a significant large impacton system performance and cost. For example, the proximity of adjacentlayers of micro-holograms in adjacent layers can result in cross talkbetween micro-holograms. This problem intensifies as the number oflayers in the disc increases.

FIG. 28 shows a format 2800 to overcome data discontinuities betweendifferent layers by storing the data in spirals in both radialdirections on a media, such as a rotatable disc. Micro-holograms arestored on one layer 2810 in a spiral that traverses inward, for example.At the end of this layer 2810, the data continues with minimalinterruption by focusing onto another layer 2820 in the disc in a spiralthat traverses in an opposite direction. Adjacent layers, e.g., 2830,may continue to alternate in starting position and direction. In thismanner, the time it would otherwise take for the sensor head to go backto the location where the previous spiral 2810 started is eliminated. Ofcourse, if it is desired to start at the same starting point as theprevious spiral, data can be stored ahead of time and read out at thedesired system rate while the detector moves back to starting point.Alternatively, different groups of layers may have one startinglocation, and/or progressing direction, while other groups of layershave another starting location and/or progressing direction. Reversingdirection of the spiral in adjacent layers may also reduce the amount ofcrosstalk between layers by providing a separation between spirals thatprogress in a same direction.

Referring now also to FIG. 29, crosstalk may be further reduced bychanging the phasing or starting point of each spiral. FIG. 29 shows aformat 2900 that includes multiple potential micro-hologram trackstarting/ending points 2910A-2910G. It should be recognized that whileeight (8) track starting/ending points are shown, any suitable number,greater or less, may be used. According to an aspect of the presentinvention the phase or starting/ending point of each layer may bealternated. Cross-talk between layers may be reduced by varying theending points of data spirals on different layers. That is, where afirst layer starts at point 2910A and spirals inward to point 2910H, anext layer may start at point 2910H and spiral outward to point 2910D,where the next layer that spirals inward then starts, for example. Ofcourse, other particular groupings of starting/ending points may beused.

Thus, micro-holograms may be stored in layers in spiral tracks thatspiral in different directions on different layers in order to reducetime needed for a read/write detector head to move to the next spiral,e.g., starting point for a next layer. During the interval when thedetector head moves from one layer to another, one or more data memoriesmay be used to maintain a consistent data stream to the user or system.Data stored in this memory from the previous data layer may be read outwhile the detector head moves to the next spiral layer. Cross-talkbetween layers may be reduced by reversing of spirals on adjacent ordifferent layers. Cross-talk between layers may also be reduced bychanging the phase or starting point of each layer and varying theending points of data spirals on different layers. The starting andending points on different layers to be read consecutively may be spacedso to avoid unnecessary or extended interruption of data during the timerequired to focus on the next consecutive layer of data.

In one embodiment, oblong shaped micro-holograms are used as the formatfor a volumetric data storage system. In other words, self trackingmicro-holograms are provided. Advantageously, using oblong shapedmicro-holograms may allow for micro-hologram size to be smaller than arecovery laser spot size in at least one lateral dimension. For trackingpurposes, the oblong shaped micro-holograms are used to determine thetrack orientation by detecting the reflection shape. A differentialsignal based on the reflected light may be used to increase systemrobustness.

Referring now also to FIG. 30, in a single-bit holographic storagemedium, format micro-holograms may be written by locally modulating therefractive index in a periodic structure the same way as data holograms.The micro-hologram generates a partial reflection of a reading laserbeam. When there is no micro-hologram, the reading laser transmitsthrough the local area. By detecting the reflected light, a drivergenerates a signal indicative of whether the content is a 1 or 0. In theillustrated case of FIG. 30, a bit is a substantially circularmicro-hologram 3010, with a size determined by the writing laser spotsize. Because the micro-hologram writing process follows the Gaussianspatial profile of the laser, the micro-holographic bit is also Gaussianin spatial profile. Gaussian profiles tend to have substantial energyoutside the light beam waist (or spot diameter). In order to decreasethe interference from the neighboring bits (micro-holograms 1, 2, 3, 4and 5), the bit separation (the distance between two bits dt) may needto be as large as three times the laser spot size. As a result, thecontent density on a layer may actually be much less than the contentdensity on CD or DVD layer. Another possible drawback associated with acircular format is associated with tracking, where a media disc isspinning in direction 3020. Referring still to FIG. 30, it is desirablethat the laser spot move to bit 2 after reading bit 1. However, sincemicro-hologram bit 1 is symmetric, the drive does not have additionalinformation to indicate the direction of the track 3030 including bits 1and 2. Accordingly, the drive may cause the laser to wander to anothertrack 3040, 3050, e.g., bit 4 or 5 unintentionally.

Referring now also to FIG. 31, to assist in correcting for potentialtrack misalignment, the micro-hologram spot shape can be madenon-circular, or non-symmetric, so that the laser head can determine thetrack orientation. In order to have a bit separation smaller than theread laser spot size 3110 in at least one lateral dimension,oblong-shaped micro-holograms 3120 having a high reflectivity are formedalong the tracks 3130, 3140, 3150. It is worth noting that in contrast,single layer formats, such as CD and DVD, use oblong shaped pits thatgenerate interference resulting in areas of relatively low reflectivity.In order to write a format as shown in FIG. 31, a media disc is spunalong the track (e.g., 3130) and a writing laser is turned on and off,depending upon whether a reflection is or is not desired in a localvolume. In other words, the media is advanced relative to the laser spotduring exposure thereby exposing an elongated portion of the media.Oblong shaped micro-holograms are written with controlled length via thelength of time the writing laser is turned on and advancement orrotation speed. This advantageously serves to eliminate the need torapidly pulse the writing laser when writing spot-by-spot. When thereading laser is focused on an oblong shaped micro-hologram, thecircular shaped Gaussian laser spot has more strength of reflectionalong the track orientation than normal to the track orientation. Thesignal reflected by the micro-hologram is no longer perfectly circular(see, e.g., FIGS. 25A-25C), and a detector, such as a quadrant detector,may be used to determine the reflected light beam shape and hence trackdirection—which is then used as a feedback to help keep the laser headon track. To increase the system sensitivity, conventional CD/DVD formatmethodologies, such as by using differential signals based onreflection, may also be incorporated.

Thus, in one embodiment oblong shaped micro-holograms are provided alongthe track inside the medium for the volumetric data storage physicalformat. The format micro-holograms may encode data themselves, oradditional data optionally recorded at different locations, orco-located yet recorded at a different angle, and/or at a differentwavelength than primary data-indicative micro-holograms. Where therecording media provides a non-linear optical response (i.e., athreshold response), the width (short dimension) of the oblong marks mayfurther be decreased thereby further increasing layer capacity.

It should be understood that the formatting systems and methodsdescribed herein are not limited to volumetric storage systems andmethods using non-linear and/or threshold responsive materials, butinstead have broad applicability to volumetric storage systems andmethods in general, including those using linearly responsive materials,such as that described U.S. Patent Publication 20050136333, the entiredisclosure of which is hereby incorporated by reference.

Formatting for Rotatable Volumetric Disc using Separate HolographicComponents

Alternatively, or in addition to self tracking data-indicativemicro-holograms, separate tracking elements may be incorporated into themedia. Without active focusing to maintain the laser spot focused to thecorrect layer and to keep the laser head on the right track, it mayprove commercially impractical to store micron or sub-micron sizefeatures inside a media disc, due at least in part to physicallimitations including, but not limited to, surface roughness andscratches.

Single layer storage formats (e.g. CD, DVD) use a reflective asymmetriclight beam for focusing, and a three-light beam mechanism for tracking.However, volumetric storage media don't include a highly reflectivelayer at the data recording levels in the medium. In recordable orre-writable versions of CD and DVD formats, tracks or grooves arepre-formed, so that the laser head follows the track when writing thedigital content. U.S. Published Patent applications 2001/0030934 and2004/0009406, and U.S. Pat. No. 6,512,606, the entire disclosures ofeach of which are hereby incorporated by reference as if being set forthin their entirety herein, propose to pre-form tracks inside a single bitholographic medium, so that a laser head can follow it in the contentwriting process. This track is also followed by the laser head duringthe reading process.

In one embodiment, track pre-formatting and/or off-axis micro-hologramsare used to encode tracking data (e.g., depth and radius positioninformation). More particularly, prior to storing micro-holographic bitsinside a volumetric storage media, tracks encoded with off-axismicro-holographic gratings are pre-recorded at various depths andpositions in the media. Such tracking micro-holograms may be oriented soas to generate a reflection off of the normal of an impinging laserbeam. The orientation angle may correlate to the tracking micro-hologramdepth and radius, such that the tracking micro-holograms serve as checkpoints. In a reading or writing process, the tracking micro-hologramsreflect incident light away from the optical normal axis, which can bedetected using a separate detector, for example. The focusing depth andradius of the current location in the disc is determined based ondetection of the angled, off-axis reflections. Pre-formedmicro-holograms may thus be used to provide a feedback signal to thedrive about the optical head position.

Precise positioning stages and a writing laser are suitable for writingtracks inside the holographic media. Each track may spiral throughvarious radii and/or depths inside the media. Of course, otherconfigurations, including circular or substantially concentric tracks,may be used though. Digital bits are written by forming micro-hologramsalong each track. A track may be formed, for example, by focusing a highpower laser to locally alternate the refractive index of the medium. Thelocally refracted index modulation generates a partial reflection fromincident focused light to a tracking detector and provides informationabout the track. Conversely, the tracks may be written into aholographic master and optically replicated into the media devices (e.g.discs), as discussed herein.

FIG. 32 shows a medium 3200 in the form of a disc may be spun to cause awriting or reading head to follow a pre-programmed track. A laser headsubstantially adjacent to the medium focuses a light beam 3210 to alocal area to facilitate writing of the track in the medium. Light beam3210 is normal to medium. Formed micro-holograms are used to encodetrack positions as off-axis angles. A second laser beam 3220 impingingfrom another side of the medium illuminates the same volume as laserbeam 3210. Light beam 3220 is off-axis from the disc normal axis. Thetwo light beams 3210, 3220 interfere and form a micro-hologram 3230off-axis from the medium normal. This off-axis angle may be used toencode the physical or logical position of the track, i.e., depth orradius. As will be understood by those possessing an ordinary skill inthe pertinent arts, the off-axis angle φ of micro-hologram 3230 isdependent upon the off axis angle φ of light beam 3220, where light beam3210 is normal to the medium 3200. Thus, by altering the angle ofimpinging light beam 3220, the location of the formed hologram may beencoded.

Light beam 3210 may take the form of a continuous wave to write acontinuous track, or be pulsed. Where pulsed, the pulse repetition ratedetermines how frequently track position can be checked during contentwriting and/or reading. Alternatively, or in addition thereto,micro-hologram bursts with varied repetition rates or numbers of pulsesmay be used in addition or in lieu of angle dependence, to encode trackposition information. However, where pulsing of the micro-hologramwriting light beam is used, such that the pulse repetition rate ornumber of pulses indicates the track position, more than one trackingmicro-hologram may need to be read to determine useful positioninginformation.

Returning again to using angular dependence, during the content writingand reading process, pre-formed off-axis micro-holograms 3230 reflect anincident laser beam 3210′ normal to the media off-axis, to provideinformation about the track. Other information, such as copyrightinformation, may optionally be encoded. In such a case, the off-axislight beam may be modulated to encode such other data, and at an angleindicative of the position within the media. Referring now also to FIG.33, when an incident light beam 3210′ normal to the media axis isfocused to a locally pre-written tracking micro-hologram 3230, thetracking micro-hologram 3230 partially reflects the light as a lightbeam 3310 having an analogous direction and spatial profile as thesecond laser beam used in the micro-hologram recording process (e.g.,light beam 3220, FIG. 32). An off-axis sensor, or array of sensors, maybe used to detect the reflected angular light beam 3310 and determinesthe position of the focused spot of the incident light beam 3210′.

Thus, track and/or other information may be encoded in pre-formed,off-axis micro-holograms. Where the off-axis angle light beam is used asan encoder, an optical drive can determine the position of the focusedincident light beam by reading a single tracking micro-hologram. Theinformation gathered may be used for focusing and tracking, e.g.,provided to a focus/tracking system akin to that shown in FIG. 27. Forexample, the off-axis signal may be used to determine whether theincident light is at the appropriate depth and whether the appropriatelens is being used to correct the spherical aberration associated withthe depth.

In one embodiment, one or more micro-holograms may include off-axisand/or off-center components. Referring now also to FIG. 34A, aholographic diffraction unit, such as a phase mask or grating, splits anincident light beam into a main light beam 3410 for writing/reading andat least one off-axis light beam for tracking 3420. The off-axis lightbeam's 3420 propagation angle θ is in-line with an off-axis, off-centertracking micro-hologram 3430 in a media 3400, such that the reflectedlight beam propagates back along the direction of the incident off-axislight beam 3420. In this scenario, additional collecting optics otherthan the objective lens may not be needed. However, the off-axis angle Eof the micro-hologram 3430 is fixed and use of the micro-hologram pulserepetition rate or pulse number modulation may be necessary to index thetrack position.

FIGS. 32-34A illustrate one off-axis micro-hologram. Alternatively, thedata micro-hologram may be formatted with two off-axis micro-holograms,one on each side. The writing of the 3 overlapping micro-holograms areshown in FIG. 34B. The micro-hologram data is written by the referencebeam 3440 and the data beam 3450, which is counter-propagating along thesame axis as the reference beam. Two off-axis micro-holograms may bewritten by the interference between the same reference beam 3440 and theoff-axis writing beams 3460, 3470.

In the read process (FIG. 34C), the reference beam 3440′ serves as theread beam. The three micro-holograms have already been stored in onelocation. The reference beam 3440′ will thus be diffracted in threedirections: the back reflection 3482 from the data micro-hologram, andthe side reflections 3484, 3486 from the two off-axis micro-holograms.When the plane formed by the two side reflections is perpendicular tothe micro-hologram data track direction, the two side reflection as anindicator for tracking.

It should be understood that the tracking and focusing systems andmethods described herein are not limited to volumetric storage systemsand methods using non-linear and/or threshold responsive materials, butinstead have broad applicability to volumetric storage systems andmethods in general, including those using linearly responsive materials,such as that described U.S. Patent Publication 20050136333, the entiredisclosure of which is hereby incorporated by reference.

Pre-Recorded Media Batch Replication

Optical replication is well suited for distributing large volumes ofdigital information recorded as micro-holograms in a supporting media.Industrial processes for optical replication using micro holographic, asopposed to page-based holographic, approaches appear desirable. Oneproblem with optical replication using linear materials is that anyundesired reflection in the optical replication system will produce anundesired hologram. Because high power lasers are typically involved inoptical replication, those undesired holograms may significantly disturbthe data indicative and/or formatting holograms. Also, the strength ofthe holograms recorded in linear materials will be directly proportionalto the ratio of the power densities of the recording laser beams. Forratios very different from 1, holograms will be weak and a largequantity of dynamic range (recording capability of the material) will beundesirably consumed. Again, this can be addressed through the use of anon-linear optically responsive media.

Referring now to FIGS. 35, 36 and 37, there are shown implementations ofoptical replication techniques suitable for use with a non-linearoptically responsive media. FIG. 35 illustrates a system for preparing amaster media, FIG. 36 illustrates a system for preparing a conjugatemaster media and FIG. 37 illustrates a system for preparing a copymedia, e.g., for distribution. Referring first to FIG. 35, there isshown a system 3500 for recording a master media 3510. In theillustrated embodiment, master media 3510 takes the form of an opticallynon-linear responsive material molded disc, such as those describedherein. Master holographic media 3510 is recorded by forming an array ofmicro-holograms 3520, one-by-one. System 3500 includes a laser 3550optically coupled to beam-splitter 3552. Laser 3550 may take the form ofa 532 nm, 100 mW CW, single-longitudinal-mode, intra-cavity doubling,diode pumped solid state Nd:YAG laser, where beam-splitter 3552 takesthe form of a polarizing cube beam splitter, for example. Focusingoptics 3532, 3542 are used to focus the split light beams 3530, 3540 tocommon volumes within media 3510, where they counter-propagate,interfere and form fringe patterns, inducing micro-hologram formation,as discussed hereinabove. Focusing optics 3532, 3542 may take the formof high numerical aperture aspheric lenses, for example. A shutter 3554is used to selectively pass light beam 3530 to media 3510, to encodedata and/or facilitate the orderly formation of micro-holograms 3520.Shutter 3554 may take the form of a mechanical, electro-optical oracousto-optical shutter having an around 2.5 ms window time, forexample.

To enable micro-holograms to be formed in particular target volumes,focusing optics 3532, 3542 are actuated to selectively focus todifferent radii from a center of spinning media, e.g., disc, 3510. Thatis, they laterally translate the focus region at different radii from acenter of spinning media, e.g., disc, 3510. The media 3510 is supportedby a precision positioning stage 3556 that spins the media, and allowsfor vertical alignment of the focused light beams 3530, 3540 atdifferent vertical layers in the media 3520. Angular positioning iscontrolled by selectively opening shutter 3554 at appropriate times. Forexample, a stepper motor or air bearing spindle may be used to rotatemedia 3510, such that the shutter may be selectively opened and shut atvarious times corresponding to different angular positions of rotatingmedia 3510.

Referring now to FIG. 36, there is shown a block diagram of a system3600. System 3600 includes a light source 3610. Light source 3610 maytake the form of a 532 nm, 90 W, 1 kHz repetition rate pulsed Nd:YAGlaser, such as the commercially available Coherent Evolution model 90,for example. Source 3610 illuminates master media 3510 through conjugatemaster media 3620. In the illustrated embodiment, conjugate master media3620 takes the form of an optically linear responsive material moldeddisc, such as that described in U.S. Patent Publication 20050136333, theentire disclosure of which is hereby incorporated by reference herein.By rapidly exposing master 3510 to source 3610 emissions 3615 throughconjugate master 3620, reflections from master 3510 interfere withemissions directly from source 3510 to form fringe patterns in conjugatemaster 3620. The holographic patterns formed in conjugate master 3620are not identical to that of master 3510, but are instead indicative ofreflections there from. According to an aspect of the present inventionentire master and conjugate master 3510, 3620 pairs may be flash, orbatch, exposed at once. Alternatively, emission 3615 may mechanicallyscan the master/conjugate master pair, as indicated by transverse arrow3618.

FIG. 37 shows a system 3700. Like system 3600, system 3700 includes alight source 3710. Source 3710 may take the form of a 532 nm, 90 W, 1kHz repetition rate pulsed Nd:YAG laser, such as the commerciallyavailable Coherent Evolution model 90, for example. Source 3710illuminates conjugate master 3620 through distribution media 3720. Inthe illustrated embodiment, media 3720, like master media 3510 andconjugate master media 3620, takes the form of an optically non-linearresponsive material molded disc, such as those described herein. Moreparticularly, source 3710 emits emissions 3715 through distributionmedia 3720 and into conjugate master media 3620. The refractive indexchanges therein, which correspond to reflections from micro-hologramarray 3520 (FIGS. 35, 36), generate reflections. These reflections againtraverse distribution media 3720, where they interfere with thecounter-propagating emissions 3715 to form interference fringe patternsindicative of a micro-hologram array 3730. Where light emissions 3715and emissions 3615 are substantially identical in direction andwavelength, array 3730 corresponds to array 3520 (FIGS. 35, 36)—therebyduplicating master 3510 as distribution media 3720. The entire conjugatemaster and distribution media 3620, 3720 pairs may be flash, or batch,exposed at once. Alternatively, emissions 3715 may scan the conjugatemaster/distribution media pair, as indicated by transverse arrow 3718.

It should be understood that systems 3500, 3600, and 3700 are onlyexamples, and several variations in setup would lead to similar results.Further, the master, conjugate master, and the distribution medium donot need to be made of the same material and can be made of acombination of linear and non-linear materials. Alternatively, they mayall be formed of a threshold responsive material, for example.

Referring now also to FIG. 38, in a different implementation 3800, themaster from which distribution media 3810 ultimately are created maytake the form of a tape, having apertures, or holes, or at leastsubstantially transparent regions. Alternatively, the master from whichdistribution media 3810 ultimately are created may take the form of aspatial light modulator, having a two-dimensional array of pixels orapertures. Either way, system 3800 includes a laser 3820, that may takethe form of a 532 nm, Q-switched, high power (e.g., 90 W, 1 kHzrepetition rate pulsed) Nd:YAG laser, such as the commercially availableCoherent Evolution model 90, for example. Laser 3820 is opticallycoupled to a beam-splitter 3830, which may take the form of a polarizingcube beam splitter, for example. Beam-splitter 3830 thus produces firstand second light beams 3830, 3840, that counter-propagate withinparticular volumes of media 3810 in a manner suitable for forming anarray of micro-holograms 3815 indicative of stored data as discussedherein. More particularly, light beam 3840 is communicated throughconditioning optics 3845 into media 3810. Light beam 3850 iscommunicated through conditioning optics 3855 into media 3810.

Conditioning optics 3845, 3855 may take the form of micro-lens array(s)suitable for transforming the laser beam into a series, ortwo-dimensional array, of focused spots. Where the lenses have a highnumerical aperture, dense packing may be realized by moving the media insmall enough increments that the exposures generate an interlaced array.Conditioning optics 3845, 3855 thus focus counter-propagating lightbeams 3840, 3850 into a two-dimensional array of focused points within asingle layer of media 3810. According to an aspect of the presentinvention, this array of points corresponds to an array of digital 0'sor 1's being recorded throughout the entire layer. Thus, by activatinglaser 3850, a layer of all digital 0's or 1's may be recorded in asingle layer of media 3810 by the interfering fringes of the spotsforming an array of micro-holograms therein. This may be of particularuse where the media takes the form of an optically non-linear responsivematerial disc, as has been described herein.

According to an aspect of the present invention, tape or spatial lightmodulator 3860 may be used to provide for different data being recordedin a single layer of media 3810. Tape or spatial light modulator 3860may include a series or array of apertures, or holes. The presence orabsence of an aperture may correspond to the digital state ofcorresponding digital data. That is, areas lacking apertures selectivelyblock light beam 3840 depending upon whether a micro-hologram is to berecorded or not, depending upon a corresponding data state.

In either case, one layer of data is recorded at a time and only in onearea of the recording medium. Medium 3810 may be advanced or rotated afew times to record a full layer, using a positioning stage 3870, forexample. The medium may be moved up or down, to record other layers,using positing stage 3870 as well, for example.

Thus, flood illumination of a master medium to record an intermediate orconjugate master may be used. Flood illumination of a master orconjugate master to record data in a distribution media may also beused. A tape or spatial light modulator may be used as a master torecord distribution media. And, diffraction efficiency (strength) ofrecorded holograms may be independent from the ratio of the recordinglaser beam power densities.

Pre-formatted Media

As set forth, holographic media discs may be recorded with arrays ofmicro-holograms indicative of a data state. These arrays may be spreadthroughout substantially all of the volume of a medium made of anoptically non-linear or threshold responsive recording material. In oneembodiment, particular data (e.g., alternating states of data) arerecorded in the pre-formatted media by erasing or not-erasing certainones of the micro-holograms. Erasing may be effected by using a singlelight beam with enough focused energy to bring the volume of themicro-hologram above the threshold condition, e.g., heating to approachTg of a constituent polymer matrix.

More particularly, recording of data into a pre-formatted medium (e.g.,an array of micro-holograms indicative of a single data state, e.g., all0's or all 1's, within an optically non-linear responsive material), maybe accomplished by either erasing or not erasing select ones of thepre-recorded, or pre-formatted, micro-holograms. A micro-hologram may beeffectively erased by focusing one or more laser beams there-upon. Wherethe light beam delivered energy exceeds the writing threshold intensity,as discussed herein-above, the micro-hologram is erased. Thus, thethreshold condition may be the same needed to be satisfied to form thetargeted micro-hologram in the first place. The light beam may emanatefrom a conventional diode laser, similar to those conventionally used inCD and DVD technologies. FIG. 39 shows a system 3900 where data isrecorded by a single laser beam, by focusing on pre-providedmicro-holograms in a pre-formatted array and selectively erasing thosemicro-holograms corresponding to a bit to be written.

More particularly, laser beam 3910 is focused by focusing optics 3920 toa target volume 3940 in a media 3930 containing a pre-formedmicro-hologram (not shown). The actual mechanism that erases thetargeted hologram may be analogous to that used to form it in the firstplace. For example, pre-formatted holograms can be erased by using asingle incident beam to cause any previously unaffected portion of thevolume element (i.e., the regions in between the original fringes) toexperience an index change resulting in the destruction of the fringepattern—thus producing a region of continuous refractive index. Further,the laser need not be single-longitudinal-mode, because no interferenceis required, making the reading and recording lasers of amicro-holographic data device advantageously simple and potentiallyrelatively inexpensive.

Optionally, a serial number may be optically recorded in the media. Thisserial number may be used to track the ownership of the recordable mediato facilitate copyright protection, for example. The serial number maybe optically recorded in a manner to facilitate optical detectionthereof. The serial number may be optically recorded in predeterminedlocation(s) in the media prior to, substantially simultaneously with, orafter, data replication using a spatial light modulator.

Such a pre-formatted non-linear recording format for a micro-holographicdata storage configuration may facilitate low cost micro-holographicrecording systems to be realized. With optics on a single side of themedium, simplified optical heads may also be used. Further, a nonsingle-longitudinal-mode laser may be used for recording data. Also,since only a single light beam is used, vibration tolerant recordingsystems for micro-holographic systems may also be realized.

It should be understood that the pre-format systems and methodsdescribed herein are not limited to volumetric storage systems andmethods using non-linear and/or threshold responsive materials, butinstead have broad applicability to volumetric storage systems andmethods in general, including those using linearly responsive materials,such as that described U.S. Patent Publication 20050136333, the entiredisclosure of which is hereby incorporated by reference.

Recovering Micro-hologram Stored Data

FIG. 40 shows a system 4000. System 4000 is suitable for detecting thepresence or absence of a micro-hologram at a particular location withina medium, such as a spinning disc media. System 4000 may be targeted toselect volumes using the tracking and focusing mechanisms describedherein. In the illustrated embodiment, a laser beam 4010 is focused by afocusing optics 4020 to impinge a target volume 4030 within a media disc4040, through a beam-splitter 4050. Light beam 4010 may emanate from aconventional laser diode, such as those used in CD and DVD players. Sucha laser may take the form of a GaAs or GaN based diode laser, forexample. Beam-splitter 4050 may take the form of a polarizing cube beamsplitter, for example. Focusing optics 4020 may take the form of highnumerical aperture focusing objective lensing, for example. Of course,other configurations are possible.

Regardless of the particulars, where a micro-hologram is present intarget volume 4030, light beam 4010 is reflected back though optics 4020to beam-splitter 4050. Beam-splitter 4050 re-directs the reflection to adetector 4060, which detects the presence or absence of a reflection.Detector 4060 may take the form of a photo-diode, surrounded by aquadrant detector, such as the commercially available Hamamatsu Si Pinphotodiode model S6795, for example.

It should be understood that the data recovery systems and methodsdescribed herein are not limited to volumetric storage systems andmethods using non-linear and/or threshold responsive materials, butinstead have broad applicability to volumetric storage systems andmethods in general, including those using linearly responsive materials,such as that described U.S. Patent Publication 20050136333, the entiredisclosure of which is hereby incorporated by reference.

Revenue Protection

Pirating, and even casual copying, of pre-recorded optical mediarepresents a significant source of economic loss for the entertainmentand software industries. The availability of recordable media withhigh-speed (such as up to 177 Mbps) data transfer rates makes itreasonably easy to copy CDs or DVDs containing copyrighted music orfeature films. In the software industry, content providers often useproduct activation codes to attempt to curtail the pirating of software.However, product activation codes and the data on the disc are notuniquely connected and several copies of the software can be installedon numerous machines with little or no way to detect the multiple copiesor preventing simultaneous use.

In conventional pre-recorded optical media, e.g., CD or DVD,pre-recorded content is conventionally replicated by stampingcorresponding data into the media during an injection molding process.This process may be used to reproduce the data on tens of thousands ofdiscs from a single master, which inherently limits the ability touniquely identify an individual disc. Several attempts have been made toprovide additional equipment and processes to mark each disc subsequentto the molding process. However, these processes typically require oneto record new data on, or erase data from, a molded disc to mark thedisc. For example, attempts have been made to use a high power laser to“mark” the disc in a way that can be read by the drive. However, thedata on the disc is considerably smaller than the spot that the laser isfocused to, such that these marks are typically larger than the data andnot easily interpreted by the drive.

Further, conventional optical data storage devices, such as DVD's, usedto distribute pre-recorded content typically have sufficient capacityfor, at most, two full length feature films. Often, content providersuse the capacity to accommodate two different viewing formats of a samecontent, for example a traditional 4:3 format combined with the 16:9format popular on more recent models of televisions.

Single-bit micro-holographic systems according to the present inventionmay be used to offer multiple, such as up to more than 50 individualfeature films on a single CD-size disc, for example. In one embodiment,each disc is marked with an individually unique identification number,or a substantially unique identification number, that is embedded in thedata and readable by the holographic drive. This is facilitated by thefact that the holographic data may be replicated in an optical manner.The ability to uniquely identify each large capacity disc enables a newbusiness model for delivering content, in which each disc can containnumerous feature films grouped by various categories (such as genre,director, lead actor or actress), for example.

In such an embodiment a consumer may acquire, such as by purchasing, apre-recorded disc. The cost may be commensurate with conventional mediathat provides user access to one content feature, such as one featurefilm, for example. According to an aspect of the present invention, theconsumer may subsequently activate, such as by purchasing, additionalcontent, such as additional feature films, contained on the disc. Thismay be accomplished by a content provider issuing an individual accesscode associated with an identification number encoded on a particulardisc, or discreet set of discs. Where the disc serial number is notcopy-able, the access code is not suitable to enable viewing of piratedcontent on another, differently serialized disc.

Further, consumers may be encouraged to copy discs (e.g., by recoveringthe data and re-reproducing it in another analogous media disc) andreceive their own access codes based on serial numbers embedded onpre-formatted recordable discs, for example. In this way, user to usercontent distribution may actually be encouraged, while preserving arevenue stream for the content owner.

In one embodiment, single-bit micro-holographic data may be reproducedfor mass-distribution by injection molding blank discs and subsequentlytransferring the data to discs through optical replication, e.g., flashexposure, as is discussed herein. Several locations on the disc may beintentionally left blank during the initial exposure of the data to bereproduced. These locations are subsequently recorded via additionaloptical exposures corresponding to identification numbers, where eachnumber is unique to each disc or set of discs using a spatial lightmodulator, for example. These locations can also be used for identifyingnumbers on blank, pre-formatted discs.

Based upon anticipated storage requirements and storage capacities, acontent-containing micro-holographic disc the size of a conventional CDmay contain up to 50 standard definition full-length feature films, or10 high definition (HD) full-length films, by way of non-limitingexample only. The content may be grouped in any number of ways. Forexample, the content provider might place films in a given series on adisc, or films with a specific leading actor or actress, or films thatfall within the same genre. The serial number of the disc may beindicated on or in the packaging of the disc when prepared for retailsale. When a consumer purchases the disc, the package may include anaccess code that the user is prompted to enter when playing the disc.The access code corresponds to the associated serialized disc to enablethe user to view one, and only one specific feature (or discrete set offeatures) on the disc. Alternatively, a player for the disc may beequipped with hardware/software to enable it to communicate with a useauthority, that provides an activation code to the player responsivelyto the serial number, and possibly the player's, identifiers and thelevel of access currently permitted.

Regardless, the drive or reading device may include memory, such assolid-state or magnetic memory devices, to store the access code once ithas been entered so subsequent viewing of the feature will not requirere-entering the number.

The user may contact the content provider, or its agent, via a computernetwork, such as the Internet, or via phone (for example via a toll-freephone call) to obtain additional activation codes that correspond toother features contained on the disc. Alternatively, the player mayprompt the user to determine whether the user wishes to purchase theadditional content, such as upon attempted selection of the digitalcontent by the user. When the user enters another activation code, orthat code is provided by a use authority for example, the player maycheck the number against the serial number of the disc and only enablesthe feature to be played if the code and serial number correspond or areassociated. Accordingly, an access code is keyed for a specific discserial number, which is not reproducible, such that while datacorresponding to a feature on a disc may be copied, an access code thatpermits access to that feature is specific to the original disc and willnot enable copies on other discs to be played.

According to an aspect of the present invention, the content itself maybe reproduced onto a preformatted, blank media disc, for example. Thecontent provider may even encourage consumers to provide copies of thedisc to other consumers, to permit the downstream copy users to limitedaccess to the content of the disc. Each disc (preformatted andprerecorded) may be provided with a unique, or substantially unique,identifier. The serial number will not transfer during copying. A userof the copy of the original media may contact the content provider oragent, analogously to the user of the original media, and request accesscodes corresponding to, or derived from, the serial number of the copymedia disc. In this manner, the content is propagated while managing thecorresponding digital rights.

According to an aspect of the present invention, a micro-holographicreplication system may thus provide the ability to (at leastsubstantially) uniquely serialize each disc in a manner that is readableby the micro-holographic drive. Micro-holograms may be recorded inreserved area(s) of the media disc by interfering two,counter-propagating laser beams, for example. Media discs may containmultiple content, such as feature films or other content, that can beaccessed, such as by purchasing, individually.

Hardware and/or software may be used to compare access codes and serialnumbers on the discs, to see if they correspond. A memory may be used tostore access codes, so future viewing of the content does not requirere-entry of the code. A business model in which new codes can bepurchased to gain access to additional content on a disc may beprovided. Pre-serialized recordable discs on which content can be copiedand for which new access codes may be used to access the copied contentmay be provided.

Using a micro-hologram containing disc and reading drive with uniqueserial numbers and a business model enabling content to be purchasedsubsequent to the acquiring the media may provide several advantages.For example, revenue may be generated by facilitating the purchase ofadditional content already contained on a user's disc. Digital rightprotection may be enhanced via the serial numbering of both contentcontaining and recordable discs and prohibiting copying of serialnumbers. Avenues of content distribution via user copying ofcontent-containing discs and the subsequent authorization of these discsmay be provided. Multiple features films, albums, or other content maybe provided, and independently activate-able on a single disc.

It should be understood that the revenue model described herein are notlimited to volumetric storage systems and methods using non-linearand/or threshold responsive materials, but instead have broadapplicability to volumetric storage systems and methods in general,including those using linearly responsive materials, such as thatdescribed U.S. Patent Publication 20050136333, the entire disclosure ofwhich is hereby incorporated by reference.

It will be apparent to those skilled in the art that modifications andvariations may be made in the apparatus and process of the presentinvention without departing from the spirit or scope of the invention.It is intended that the present invention cover such modifications andvariations of this invention, including all equivalents thereof.

1. A data storage device comprising: a plastic substrate having aplurality of volumes arranged along tracks in a plurality of verticallystacked, laterally extending layers; and, a plurality of micro-hologramseach contained in a corresponding one of said volumes; wherein, thepresence or absence of a micro-hologram in each of said volumes isindicative of a corresponding portion of data stored.
 2. The device ofclaim 1, wherein said substrate is an about 120 mm diameter disc.
 3. Thedevice of claim 1, wherein the substrate comprises a thermoplastic. 4.The device of claim 3, wherein the thermoplastic has a non-linearfunctional characteristic.
 5. The device of claim 4, wherein thenon-linear functional characteristic is a threshold functionalcharacteristic.
 6. The device of claim 3, wherein the substrate furthercomprises a thermal catalyst.
 7. The device of claim 1, wherein saidsubstrate comprises a dye.
 8. The device of claim 7, wherein said dye isa reverse saturable absorber dye.
 9. The device of claim 1, wherein thesubstrate comprises a poly(ethylene oxide)/polystyrene block copolymer.10. The device of claim 1, wherein said substrate comprises apolycarbonate/polyester block copolymer.
 11. The device of claim 1,wherein said substrate comprises an ortho-nitrostilbene containingpolymer.
 12. The device of claim 1, wherein said substrate comprisesortho-nitrostilbene and polymethylmethacrylate.
 13. The device of claim1, wherein said substrate comprises polycarbonate.
 14. The device ofclaim 1, wherein said micro-holograms are substantially circular. 15.The device of claim 1, wherein said micro-holograms are oblong.
 16. Thedevice of claim 1, wherein said substrate is a disc having a center, atleast one of said layers spirals toward said disc center, and at leastone other of said layers spirals from said disc center.
 17. The deviceof claim 1, wherein each of said layers has a starting and ending point,and at least one of said starting points is substantially verticallyaligned with at least one of said ending points.
 18. The device of claim1, further comprising a second plurality of micro-holograms in saidsubstrate and indicative of tracking information.
 19. The device ofclaim 18, wherein each of said data indicative and second plurality ofmicro-holograms has an axis, and the axis of said data indicativemicro-holograms is distinct from the axis of said second plurality ofmicro-holograms.
 20. The device of claim 19, wherein an angle associatedwith the axis of a given one of said second plurality of micro-hologramsis indicative the position thereof in the substrate.
 21. A method forstoring data comprising: providing a plastic substrate having aplurality of volumes arranged along tracks in a plurality of verticallystacked, laterally extending layers; and, forming a plurality ofmicro-holograms in said substrate; wherein, each of said micro-hologramsis substantially contained in a corresponding one of said volumes andthe presence or absence of a micro-hologram in each of said volumes isindicative of a corresponding portion of the data.
 22. The method ofclaim 21, wherein said micro-holograms are selectively formeddependently upon the data.
 23. The method of claim 21, furthercomprising selectively erasing select ones of the micro-hologramsdependently upon the data.
 24. The method of claim 21, wherein saidforming comprises interfering two counter-propagating light beams. 25.The method of claim 24, further comprising focusing one of said lightbeams.
 26. The method of claim 25, wherein one of said beams isdivergent.
 27. The method of claim 25, further comprising reflecting oneof said light beams to provide the other of said light beams.
 28. Themethod of claim 24, further comprising selectively obscuring at leastone of the light beams dependently upon the data.
 29. The method ofclaim 21, further comprising forming a second plurality of hologramshaving a reflection direction distinct from that of said plurality ofholograms.
 30. The method of claim 29, wherein said second plurality ofholograms define said tracks.
 31. The method of claim 21, furthercomprising forming a second plurality of micro-holograms at a givenspacing, wherein the spacing is indicative of the position within thesubstrate thereof.
 32. The method of claim 21, further comprisingforming a second plurality of micro-holograms, wherein at least one ofsaid second plurality of micro-holograms is co-located in a common oneof the volumes with at least one of the plurality of micro-holograms.33. The method of claim 21, further comprising flood illuminating saidmicro-holograms through a second plastic substrate.
 34. The method ofclaim 33, wherein said illuminating induces a pattern of refractiveindex changes in said second plastic substrate.
 35. The method of claim34, further comprising flood illuminating said second plastic substratethrough a third plastic substrate.
 36. The method of claim 35, whereinsaid illuminating said second plastic substrate through a third plasticsubstrate duplicates said plurality of micro-holograms in said thirdsubstrate.
 37. The method of claim 36, wherein said flood illuminationsuse laser beams.
 38. The method of claim 37, wherein said laser beamshave a central wavelength corresponding to said micro-holograms.
 39. Themethod of claim 38, wherein said central wavelength is about 532 nm.